ALERT For Hazardous Conditions Developing Monday Night Through Mid-Morning Tuesday
Developing NW flow in combination with a pocket of very cold air aloft will generate snow, snow showers and locally intense snow squalls during Monday night into Tuesday.
Snowfall Forecast
(30 November-1 December 2020)
2″ to 4″ below 3000 feet
4″ to 8″ above 3000 feet
2″ or less in downslope locations
Locally higher amounts will be possible, especially at highest elevations within the High Knob Massif.
Target Snowfall: 3″-4″ in Norton-Wise (+/- 1″ error potential), which implies a 2″ to 5″ spread is possible for elevations below 3000 feet in the upslope zone north and northwest of the High Knob Massif.
Early Winter 2020_High Knob Massif
Introduction To The High Knob Massif
(Select 1080p HD for best viewing on YouTube)
High Knob Massif_Video And Introduction Details
Very cold air aloft and steep lapse rates (strong vertical drop in temperature with height) will support bursts of heavy snow (squalls).
Temperatures are predicted to drop to between -25 to -30 degrees Fahrenheit below zero at 17,000 to 18,000 feet during Monday Night into Tuesday Morning.
While most prevalent in upslope locations, some of these may also impact downslope locations into the Clinch, Powell, Holston and Great valleys with rapid sticking during periods of low visibility.
Heavy Snowfall Potential
A major change in the weather pattern across the continental USA is coming as December begins next week.
This also marks the start of Meteorological Winter, the three month period from December through February used for climate purposes to characterize the winter season.
This is a pattern that models have long been showing to various degrees, as evident by my weekly weather column written and submitted back on 23 November 2020:
It is now clear that a pattern supporting bursts of heavy snow embedded within a general field of light snow and snow showers will develop across the southern-central Appalachians later Monday into Tuesday of 30 November-1 December 2020.
Cold air aloft in combination with upsloping NW flow will generate widespread snow, snow showers and more intense snow squalls. As would be expected, heaviest snowfall amounts will be along the western side of the Appalachians.
Locations along and northwest of the Eastern Continental Divide, Tennessee Valley Divide and High Knob Massif, will feature the heaviest snowfall amounts versus downslope decreases in between and leeward of these major mountain barriers.
Note that this system is just coming within view of short-range, high-resolution models. At the present time (27 November), the upper air forecast would suggest that temperatures in the mid-10s to mid-20s will be common from the High Knob Massif north and northwest across Norton-Wise, Pound, Clintwood and nearby communities by the predawn-sunrise hours of Tuesday (1 December). Much lower wind chills will be felt across exposed mountain ridges and plateaus.
It is likely that Tuesday will feature little temperature rise, and may even have falls depending upon the exact timing of coldest air.
While too early to actually predict snowfall amounts with high confidence, preliminary analysis using past systems already documented in climatology suggests that upslope locations can expect 3″ to 6″ below 3000 feet, with 6″ to 12″+ above 3000 feet (with drifting on ridges and riming at upper elevations).
Tuesday is currently looking to be a mid-winter type of day, albeit with an early shot of “fresh” arctic air.
Extended Wintry Pattern
The Pacific Ocean will be looked at a great deal this winter season as La Nina acts as one of the major forcing factors for the 2020-21 Winter.
A positive (+) PNA pattern favors western USA ridging and eastern USA troughing in the upper air, with colder than average conditions across the Mountain Empire.
It currently appears that a wintry pattern will dominate the first half of December across the eastern USA. What happens during the always important second half of the month remains unclear from this distance.